1. Technical Field of the Invention
The present invention relates in general to the field of communications systems, and in particular, by way of example but not limitation, to adaptive channel filtrations that are responsive to relative levels of received signal power in wireless communications systems.
2. Description of Related Art
Mobile wireless communication is becoming increasingly important to both individuals and organizations for providing safety, convenience, improved productivity, and simple conversational pleasure to subscribers of wireless communications services. One prominent mobile wireless communication option is cellular communication. Cellular phones, for instance, can be found in cars, briefcases, purses, and even pockets. While the number of cellular phone subscribers continues to rise, the types and quality of services being demanded by cellular phone subscribers are dramatically rising as well. One type of service whose popularity has been skyrocketing, especially with the emerging prominence of e-mail, faxing, and the Internet, is data transmission.
Data may be transmitted on, for example, existing voice, data, and/or control channels. However, many of the new features that subscribers are demanding require extraordinarily high data transfer rates that cannot be effectively provided by existing channels. New mechanisms for transmitting at extraordinarily high data rates are needed to meet the demand. In accordance with the Global System for Mobile Communications ++ (GSM++) standard, a new scheme termed “Enhanced Data rates for Global Evolution” (EDGE) is under development. EDGE is intended to increase the data transfer rate available to mobile users.
EDGE systems achieve a high data transfer rate by changing the coding and modulation. Consequently, EDGE systems need a higher Carrier-to-Interference (C/I) performance ratio to achieve the intended higher data transfer rates. However, normal GSM cell planning specifications are used when designing EDGE systems so as not to reduce the total number of subscribers that may be accommodated by a given system. Because GSM cell planning specifications are reused, the requirements relating to adjacent channel rejection (C/IA1) are of the same magnitude as for an ordinary GSM system. As a result, EDGE channel filters should not only avoid disturbing the C/I-performance, but they should also be capable of meeting the ordinary GSM adjacent channel requirements.
Conventional approaches are directed to optimizing the channel filter for the best compromise between adjacent channel rejection (C/IA1) and co-channel (C/I) performance. This compromise, by definition, is optimal for only a single situation, if any. Consequently, receivers using the conventional approach are sub-optimal in most situations inasmuch as they use a fixed, pre-set compromise between adjacent channel rejection and co-channel performance.